meta: Remave meta_activate_session()

It's not used anywhere, lets drop it.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1689>

src/meta/main.h

@@ -38,9 +38,6 @@ int             meta_run                    (void);
 META_EXPORT
 void            meta_register_with_session  (void);
 
-META_EXPORT
-gboolean        meta_activate_session       (void);  /* Actually defined in meta-backend.c */
-
 META_EXPORT
 gboolean        meta_get_replace_current_wm (void);  /* Actually defined in util.c */